This invention relates to a process for the preparation of 5-alkyl-4-hydroxy-3[2H]furanones and 2,5-dialkyl-4-hydroxy-3[2H]furanones or tautomeric forms thereof, wherein the alkyl radicals independently may comprise one to six carbon atoms.
The furanones, subject of the present invention, are flavoring and perfume ingredients and have been widely utilized for the reconstitution of good quality flavors and fragrances. After their identification as volatile flavor component in nature, e.g., in pineapple (J. O. Rodin et al., J. Food Sci. 30, 280 (1965), 4-hydroxy-3[2H]furanones have found an ever increasing acceptance also based upon their versatility and applicability in a great variety of different flavor types. Their use in flavoring agents has been described in, for example, German patent application No. 1,915,788 for bakery products, Dutch patent applications No. 70 04 150 or meat flavors, German patent application No. 2,202,065 and No. 2,202,066 for tobacco flavoring, Swiss patent No. 540,650 for cooked fruit flavors and European patent application No. 0,167,214 as a sugar simulating compound. Their use as perfume ingredient is described in, for example, German patent application No. 2,509,668 and their use in insect attractant compositions is mentioned in U.S. Pat. No. 4,447,447.
For the preparation of 2,5-dialkyl-4-hydroxy-3[2H]furanones many synthetic routes have been suggested. However, all of these either are laborious non-economical laboratory methods with low overall yields, or use rare, expensive, dangerous and toxicologically undesirable methods and/or raw materials. Methods heretofore proposed for the preparation of 5-alkyl-4-hydroxy-3[2H]furanones and 2,5-dialkyl-4-hydroxy-3[2H]furanones have been described, for example, in the German patent applications No. 1,932,799, 2,163,223, 2,359,891, 2,812,713 and 2,845,843. Swiss patent No. 491,904, European patents No. 0,000,907 and 0,055,976 in Japanese patent application No. 79,115,369 and by L. Re et al., Helv. Chim. Acta 56, (6), 1882 (1973), M. Baumann et al., Synthesis 1981 (9) 709 and C. H. Wong et al., J. Org. Chem. 48 (20), 3493 (1983). These methods of preparation comprise cyclization of straight chain compounds of proper oxidation level, and oxidations of 3[2H] furanone derivatives. In German patent application 2,812,713 alkylation of sodium salts of 2-carbalkoxy-5-alkyl-3,4-dihydroxyfuran is set forth. This method requires large excesses of the alkylation agent, very long reaction periods at elevated temperatures, which is unfavorable for both purity and yield because of the general thermolability of 4-hydroxy-3[2H]furanones, water-free conditions, nitrogen atmosphere and complex systems of organic solvents. Yields are moderate to low. As an explicit condition for this method of preparation, it is claimed that the alkylation step of sodium salts of 2-carbalkoxy-5-alkyl-3,4-dihydroxyfuran is performed prior to the hydrolysis of the carbethoxy group of the resultant reaction product.